Described below is a stroke transmitter, including use in a metering valve.
FIG. 1 shows an exemplary embodiment of a known metering valve. A first piston having the surface area A1 is guided in a first cylinder and is driven directly by an actuator. A further piston having the surface area A2 is connected to a valve needle. The volume between the pistons is filled with a fluid, specifically oil in most cases, and connected to a channel. A throttle is integrated into this section in order to suppress vibrations. When the actuator moves, a certain volume of fluid, specifically x1*A1, is displaced by the first piston and also moves the second piston through the bore, where x2=x1*A1/A2. A deflection amplification, specifically the ratio of actuator stroke to needle stroke, can be achieved as a function of the chosen area ratios. Fluid can be metered quickly and precisely with the aid of a connection between a high-speed piezoelectric actuator and a stroke transmitter according to FIG. 1. This requirement will become more and more important in the future in the case of internal combustion engines that operate on gas or gasoline.
FIG. 2 shows a more detailed known stroke transmitter concept. When an actuator (not shown in the figure) is actuated, it presses a first piston, as a result of which the pressure in the hydraulic volume increases and moves a second piston. The second piston opens the metering valve, for example. On account of the operating principle of the system, oil is discharged through annular gaps. This leads to a diminishing volume over time and consequently to a loss of stroke, which is also referred to as drift. A further disadvantage is the friction between the pistons and the cylinders. This friction leads on the one hand to a stroke loss and on the other hand a piston can seize or become immovably jammed in a cylinder. The friction can be reduced in the known manner by coating the contact surfaces, usually with diamond-like carbon (DLC). However, this is time-consuming and expensive and it has so far not been possible to eliminate stroke loss as a consequence of gap flow.
FIG. 3 shows a metering valve based on a further concept for a hydraulic stroke transmitter. The hydraulic fluid is contained in hermetically sealed metal bellows which together form the hydraulic volume. The pistons are welded on as end faces. They no longer need to move in a hydraulic fluid. There is no piston friction in the hydraulic volume, and the gap flow problem is likewise eliminated. However, a concept of this kind still continues to have the following disadvantage: A change in temperature causes the volume of metal bellows to change in accordance with the associated volume expansion coefficient. The enclosed fluid has a different volume expansion coefficient. Hydraulic oils, which may be employed in this context, typically have a very high volume expansion coefficient. This leads to the undesired effect that the fluid expands to a greater extent than a metal bellows. As a result the oil needs more volume when the temperature increases. This leads to a pressure increase in the hydraulic volume 4 and ultimately to an undesired leakage when the valve is finally forced open.